Being a solid solution of a type silicon nitride, an α-sialon (Si—Al—O—N) because of its high hardness and excellent wear resistance, high-temperature strength and oxidation resistance is finding its uses in such as slide members and high-temperature resistant structural components.
The α-sialon has a structure in which atoms of a specific element (such as Ca, Li, Mg, Y, or one or more lanthanoid except La and Ce) are entered into the crystal lattice to form a solid solution while maintaining their electrical neutrality so that the Si—N bond is partly substituted with the Al—N bond (partly also with the Al—O bond). In recent years, after the discovery that by suitably selecting the element for this entry and solid dissolving, fluorescence properties which are useful for a white light emitting diode (hereinafter referred to as “white LED”) can be revealed, its putting to practical use has been under investigation (See References 1 to 5.).
A conventional α-sialon for use as a slide member, structural component or the like has been made in the form of a dense sintered body. In this case, a mass of mixed powders of silicon nitride (Si3N4), aluminum nitride (AlN) and an oxide of a solid-dissolving element is sintered in a nitrogen atmosphere under normal or gas pressure or by hot pressing or the like to form a solid solution while densifying the mass simultaneously. This is to ensure that a liquid phase formed of a surface oxide layer of silicon nitride and aluminum nitride and the oxide of the solid-dissolving element during the sintering process allows the mass densification to proceed and in the latter period of the sintering process the liquid phase is solid-dissolved within the powder grains so as not to leave a glass phase at the grain boundaries.
In the case where a mass of α-sialon powder is used as the starting material, sintering even at a temperature close to its decomposition temperature does not allow the mass to be densified well and does necessitate an assistant in order to form the liquid phase with the result that the glass phase is then left at the grain boundaries. For reasons such as that such a grain boundary glass phase is undesirable in mechanical properties, the α-sialon powder has little been used as the starting material for uses of α-sialon in slide members, structural components and the like. On the other hand, while a phosphor for white LED is used in which particles of submicron to micron size are dispersed in a sealing material such as epoxides, it is the present situation that, for the reason stated above, any α-sialon powder has never been marketed.
As a typical process of synthesizing an α-sialon powder, the reduction nitridation method can be cited in which a mass of mixed powders of aluminum oxide (Al2O3), silicon oxide (SiO2) and an oxide of a metal which is to be solid-dissolved in the lattice is heat-treated in the presence of carbon in a nitrogen atmosphere as described in References 6 to 9 below. Although this method has the feature that the α-sialon powder can be synthesized at a relatively low temperature around 1500° C. from the source powders which are inexpensive, not only a plurality of intermediate products in the synthesis process, but also the production of gas components such as SiO and CO has made it difficult to yield a product which is of single phase and to control the composition and granular size strictly. An α-sialon powder can also be obtained by firing at high temperature a mixture of silicon nitride, aluminum nitride and an oxide of an element which is to be solid-dissolved in the lattice and then pulverizing the resultant sintered body of α-sialon.    Reference 1: Japanese Patent Laid Open Application, JP 2002-363554 A    Reference 2: Japanese Patent Laid Open Application, JP 2003-336059 A    Reference 3: Japanese Patent Laid Open Application, JP 2003-124527 A    Reference 4: Japanese Patent Laid Open Application, JP 2003-206481 A    Reference 5: J. W. H. van Krebel, “On new rare-earth doped M-Si—Al—O—N materials”, TU Eidhoven, The Netherlands, p. 145-161 (1998)    Reference 6: M. Mitomo et al., “Preparation of α-Sialon Powders by Carbothermal Reduction and Nitridation”, Ceram. Int., 14, 43-48 (1998)    Reference 7: J. W. T. van Rutten et al., “Carbothermal Preparation and Characterization of Ca-α-SiAlON”, J. Eur. Ceram. Soc., 15, 599-604 (1995)    Reference 8: K. Komiya et al., “Hollow Beads Composed of Nanosize Ca α-SiAlON Grains”, J. Am. Ceram. Soc., 83, 995-997 (2000)
The conventional methods of making an α-sialon powder require a pulverization treatment under severe conditions in order to obtain the powder of a desired particle size, because intergranular bonds by liquid phase sintering in the firing process remain firm. As the pulverizing conditions become severer, the problem arises that the chances of entry of impurities are increased and also of entry of defects onto individual particle surfaces.
The use as a phosphor of an α-sialon powder as made by the conventional methods poses the problem that inasmuch as it is the particle surface area which mainly is responsive to excitation light, the defects introduced into such surface areas largely affect its fluorescent properties and cause its light emitting characteristic to deteriorate.